Thermal bondable polyolefin fibers comprising a random copolymer of propylene

ABSTRACT

The present invention relates to thermal bondable fibers comprising a random polymer of propylene with one or more olefins comonomers different from ethylene, to the process for preparing said fibers, and to the thermally bonded articles obtained from said fibers. Fibers of certain thermoplastic materials are used widely in the manufacturing of thermally bonded products.

This application is the U.S. national phase of International ApplicationPCT/EP00/02674, filed Mar. 24, 2000.

The present invention relates to thermal bondable fibers comprising arandom copolymer of propylene with one or more olefin comonomersdifferent from ethylene, to the process for preparing said fibers, andto the thermally bonded articles obtained from said fibers.

Fibers of certain thermoplastic materials are used widely in themanufacturing of thermally bonded products, such as nonwoven articles,by various processes. Said processes are mainly staplecarding/calendering, through air-bonded, spunbonding, melt-blown, andany combination of them for composite structures of nonwovens.

There have been various attempts made to improve the thermal bondability(i.e. the bond strength) of fibers and/or the calendering speed, amongwhich the use of random copolymers of propylene has been contemplated.

In particular, according to EP-A-416 620 fabric laminates having layersmade of fibers formed from olefin copolymers, terpolymers, and blends ofpolymers having a crystallinity less than 45% provide improved thermalbonding and therefore improved fabric characteristics. However thisdocument provides a concrete disclosure of propylene-ethylene copolymersonly, and points out that said copolymers produce fibers with lowertenacity and lower modulus than those formed from polypropylene.

According to U.S. Pat. No. 4,211,819 heat-melt fibers are obtained byspinning a crystalline propylene terpolymer consisting of specifiedamounts of propylene, butene-1 and ethylene. However such fibers areused as binder material only, the mechanical properties being conferredby other materials. In fact, when nonwoven fabrics are prepared in theexamples, the said fibers are mixed with rayon fibers beforecalendering.

Therefore it would be advantageous to provide fibers containing olefincopolymers and having an improved thermal bondability associated withhigh mechanical properties. In the typical process of melt spinning, thepolymer is heated in an extruder to the melting point and the moltenpolymer is pumped under pressure through a spinneret containing a numberof orifices of desired diameter, thereby producing filaments of themolten polymer. The molten polymer filaments are fed from the face ofthe spinneret into a cooling stream of gas, generally air, where thesefilaments of molten polymer are solidified as a result of cooling toform fibers.

In processes of this kind it would be advantageous to be able to operatewith the highest possible spinning speed without impairing the finalproperties of the so obtained fibers. It has now been found that all thesaid advantages are obtained by spinning specific random copolymers ofpropylene.

Accordingly, the present invention provides thermal bondable polyolefinfibers comprising 1% by weight or more, in particular 20% by weight ormore, of a random copolymer A) of propylene with one or more comonomersselected from α-olefins of formula CH₂═CHR, wherein R is a C₂-C₈ alkylradical, preferably a C₂-C₆ alkyl radical, the amount of said comonomeror comonomers being from 3% to 20% by weight with respect to the totalweight of the random copolymer A).

From the above definitions it is evident that the term “copolymer”includes polymers containing more than one kind of comonomers.

It has been unexpectedly found that the said fibers have Tenacity valuescomparable to or higher than the tenacity obtainable by spinningpropylene homopolymers under substantially the same conditions, whileachieving particularly high values of bond strength at unusually lowthermal bonding temperatures.

In particular, the thermal bondable fibers of the present invention arepreferably characterized by Tenacity values equal to or higher than 10cN/Tex (measured as explained in the examples), specially equal to orhigher than 15 cN/Tex, for instance from 10 to 60 cN/Tex or from 15 to60 cN/Tex.

Moreover, the fiber retraction tends to increase with the amount ofrandom copolymer A). This is very important to enhance the self-crimpingeffect of the fiber. The so obtained high level of self-crimping inducesbulkiness in the final nonwovens with higher soft feeling. Also thehigher softness contributes, with the soft touch, to improve the finalnonwoven quality, in particular for the hygiene applications where themarket appreciates very soft nonwovens with clothlike appearance.

Preferred amounts of α-olefins of formula CH₂═CHR (R being a C₁-C₈alkyl) in the random copolymer A) are from 5% to 16% by weight, inparticular from 5.5% to 13% by weight. Examples of α-olefins of theabove reported formula, present as comonomers in the random copolymerA), are 1-butene, 1-hexene, 4-methyl-1-pentene, 1-octene. Preferred are1-butene and 1 -hexene; particularly preferred is 1-butene.

The presence of substantive amounts of ethylene (indicatively, more than0.5-1% by weight) in the random copolymer A) is excluded; particularlypreferred is a random copolymer A) wherein the comonomer or comonomerspresent are selected exclusively from the said α-olefins of formulaCH₂═CHR, wherein R is a C₁-C₈ alkyl radical.

Preferably the Melt Flow Rate (MFR, measured according to ISO 1133 at230° C. with a load of 2.16 Kg) of the random copolymer A) used forpreparing the fibers of the present invention is within the range from 5to 2000 dg/min., more preferably from 10 to 1000 dg/min.

In the fiber the MFR of the random copolymer A) or of the polymercomposition comprising the copolymer A) can be higher, depending uponthe degree of thermal degradation occurring during the spinning process.

Such values of MFR can undergo even significative variations from thecenter to the surface of the fiber, depending upon the formation ofskin-core structures where the skin, i.e. a more or less thick layer ofpolymer on the surface of the fiber, has high MFR values caused by thesaid thermal degradation.

However it has been surprisingly found that the fibers of the presentinvention do not necessarily require the formation of skin-corestructures to achieve high levels of bond strength, even if theformation of a skin-core structure further enhances this property.

It has been found that a particularly good balance of bond strength andmechanical features is obtained when the fibers of the present inventionare prepared from random copolymers A) having values of Tensile Strengthat yield (measured according to ISO R 527) equal to or higher than 24MPa, in particular from 24 to 35 MPa, preferably equal to or higher than25 MPa, more preferably higher than or equal to 26 MPa, in particularfrom 25 or 26 to 35 MPa.

Even better properties are achieved when the fibers of the presentinvention are prepared from a polymeric material obtained by subjectingto chemical degradation (visbreaking) a random copolymer A) having thesaid values of Tensile Strength at yield, or a polymer compositioncontaining the same.

Other preferred features of the random copolymer A) used for preparingthe fibers of the present invention are:

a melting temperature from 135 to 156° C., and a crystallizationtemperature from 85 to 120° C., both measured by DSC (DifferentialScanning Calorimetry) with a temperature variation of 20° C. per minute;

fraction insoluble in xylene at 25° C. higher than or equal to 93% byweight, more preferably higher than or equal to 95% by weight;

Polydispersity Index (PI, measured with the method described in theexamples) from 2 to 5;

Flexural Modulus (measured according to ISO 178) from 500 to 1500 MPa;

Izod Impact Strength (notched) at 23° C. (measured according to ISO180/1) equal to or higher than 20 KJ/m²;

Elongation at yield (measured according to ISO R 527) from 8 to 14%;

The ratio of the value of Tensile Strength at yield to the value ofElongation at yield for the random copolymer A), either before or afterthe said polymer degradation (when occurring) is preferably from 2 to 4,more preferably from 2.1 to 4.

Particularly preferred values of Tenacity for the fibers of the presentinvention are equal to or higher than 20 cN/Tex, in particular from 20to 60 cN/Tex; most preferred are values equal to or higher than 25cN/Tex, in particular from 25 to 60 cN/Tex.

Moreover the fibers of the present invention have preferably Elongationat break values from 80% to 350%, more preferably from 100% to 250%(measured as explained in the examples).

The titre of the fibers is preferably equal to or higher than 0.8 dTex,more preferably from 1 to 10 dTex (measured as explained in theexamples). The definition of fibers according to the present inventioncomprises continuous filaments, cut fibers (staple) and short fibers(the latter being for instance obtained with the melt blown process andpreferably having lengths within the range from 5 mm to 100 mm).

The random copolymer A) belongs to the well known family of the random,crystalline or semicrystalline copolymers that can be obtained by way ofpolymerization processes in the presence of coordination catalysts. Saidprocesses and the copolymers obtained from them are widely described inthe art. For example one can use the high yield and highlystereospecific Ziegler-Natta catalysts and the polymerization processesdescribed in EP-A-45977.

The above mentioned MFR values can be obtained by adequately adjustingthe molecular weight regulating agent (such as hydrogen, for example)or, as previously said, can be achieved by way of a chemical degradationtreatment to which the polymeric material is subjected before or duringthe preparation of the fibers. An additional contribution to theobtainment of the final MFR of the polymeric material constituting thefiber can be given by the previously said thermal degradation occurringin the preparation of the fiber, particularly when the molten filamentsexit from the spinneret into the cooling zone.

The chemical degradation of the polymer chains is carried out by usingappropriate and known techniques.

One of said techniques is based on the use of peroxides which are addedto the polymeric material in a quantity that allows one to obtain thedesired degree of chemical degradation. Such degradation is achieved bybringing the polymeric material at a temperature at least equal to thedecomposition temperature of the peroxides.

Preferably, the degree of chemical degradation is from 0.9 to 0.01,expressed in terms of the ratio MFR (1) to MFR (2), where MFR (1) is thevalue of MFR before degradation, while MFR (2) is the value of MFR afterdegradation.

The peroxides that are most conveniently employable for the chemicaldegradation have a decomposition temperature preferably ranging from 150to 250° C. Examples of said peroxides are the di-tert-butyl peroxide,the dicumyl peroxide, the 2,5-dimethyl-2,5-di(tert-butyl peroxy) hexyne,and the 2,5-dimethyl-2,5-di(tert-butyl peroxy) hexane, which is marketedunder the Luperox 101 trade name.

An advantageous embodiment of the present invention is represented bythermal bondable fibers comprising a polyolefin composition C)containing from 1% to 100% by weight, preferably from 20% to 100% byweight, more preferably from 40% to 100% by weight, in particular from50% to 100% by weight, most preferably from 70% to 100% by weight of therandom copolymer A), and from 0% to 99% by weight, preferably from 0% to80%, more preferably from 0% to 60% by weight, in particular from 0% to50% by weight, most preferably from 0% to 30% by weight of a polyolefinB) (different from the random copolymer A), in particular as regards thecontent of comonomers, i.e. not falling in the previously givendefinition of random copolymer A)).

Generally, the polyolefin B) is selected from polymers or copolymers,and their mixtures, of CH₂═CHR olefins where R is a hydrogen atom or aC₁-C₈ alkyl radical.

Particularly preferred are the following polymers:

1) isotactic or mainly isotactic propylene homopolymers, andhomopolymers or copolymers of ethylene, like HDPE, LDPE, LLDPE;

2) crystalline copolymers of propylene with ethylene and/or C₄-C₁₀α-olefins, such as for example 1-butene, 1-hexene, 4-methyl-1-pentene,1-octene, wherein the total comonomer content ranges from 0.05% to 20%by weight with respect to the weight of the copolymer (said copolymersbeing different from the random copolymer A) as regards the content ofcomonomers, in particular containing less than 3%, preferably less than2.5% by weight of C₄-C₁₀ α-olefins and/or more than 1%, preferably morethan 2% by weight of ethylene), or mixtures of said copolymers withisotactic or mainly isotactic propylene homopolymers;

3) elastomeric copolymers of ethylene with propylene and/or a C₄-C₁₀α-olefin, optionally containing minor quantities (in particular, from 1%to 10% by weight) of a diene, such as butadiene, 1,4-hexadiene,1,5-hexadiene, ethylidene-1-norbornene;

4) heterophasic copolymers comprising (I) a propylene homopolymer and/orone of the copolymers of item 2), and an elastomeric fraction (II)comprising one or more of the copolymers of item 3), typically preparedaccording to known methods by mixing the components in the molten state,or by sequential polymerization, and generally containing theelastomeric fraction (II) in quantities from 5% to 80% by weight;

5) 1-butene homopolymers or copolymers with ethylene and/or otherα-olefins.

Moreover, the fibers of the present invention may be single(monocomponent) fibers (i.e. substantially made of the said randomcopolymer A) or of a composition comprising the random copolymer), likethe said composition C)) or composite fibers (i.e. comprising one ormore additional portions arranged symmetrically or asymmetrically, forinstance side-by-side or sheath-core, comprising various and differentkinds of polymeric materials). Preferred examples of polymeric materialsthat can constitute or be present in the said additional portions arepolyethylene, polyisobutylene, polyamides, polyesters, polystyrene,polyvinyl chloride, polyacrylates and mixtures thereof.

The fibers of the present invention can contain formulations ofstabilizers suited for obtaining a skin-core structure (skin-corestabilization), or a highly stabilizing formulation. In the latter case,a superior resistance to aging is achieved, for durable nonwovens.

Preferred examples of skin-core stabilizations are those comprising oneor more of the following stabilizers (percent by weight with respect tothe total weight of polymer and stabilizers):

a) from 0.01% to 0.5% of one or more organic phosphites and/orphosphonites;

b) from 0.005% to 0.5% of one or more HALS (Hindered Amine LightStabilizer);

and optionally one or more phenolic antioxidants in amounts not higherthan 0.02%.

Specific Examples of Phosphites Are:

tris(2,4-di-tert-butylphenyl)phosphite marketed by CIBA GEIGY under thetrademark Irgafos 168; distearyl pentaerythritol diphosphite marketed byBORG-WARNER CHEMICAL under the trademark Weston 618; 4,4′-butylidene bis(3-methyl-6-tert-butylphenyl-di-tridecyl) phosphite marketed by ADEKAARGUS CHEMICAL under the trademark Mark P; tris(monononylphenyl)phosphite; bis(2,4-di-tert-butyl) pentaerithrytol diphosphite,marketed by BORG-WARNER CHEMICAL under the trademark Ultranox 626.

A preferred example of phosphonites is the tetrakis(2,4-di-tert-butylphenyl) 4,4′-diphenylilenediphosphonite, on whichSandostab P-EPQ, marketed by Sandoz, is based. The HALS are monomeric oroligomeric compounds containing in the molecule one or more substitutedamine, preferably piperidine, groups.

Specific examples of HALS containing substituted piperidine groups arethe compounds sold by CIBA-GEIGY under the following trademarks:

Chimassorb 944 Chimassorb 905 Tinuvin 770 Tinuvin 292 Tinuvin 622Tinuvin 144 Spinuvex A36

and the product sold by American CYANAMID under the trademark Cyasorb UV3346. Examples of phenolic antioxidants are:tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)-s-triazine-2-4-6-(1H,3H, 5H)-trione, marketed by American CYANAMID under the trademark Cyanox1790; calcium bi [monoethyl(3,5-di-tert-butyl-4-hydroxy-benzyl)phosphonate];1,3,5-tris(3,5-di-tert-butyl4-hydroxybenzyl)-s-triazine-2,4,6(1H, 3H,5H) trione; 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl4-hydroxybenzyl)benzene; pentaerythrityl-tetrakis [3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]; octadecyl 3-(3,5-di-tert-butyl4-hydroxyphenyl)-propionate,marketed by CIBA GEIGY under the trademarks Irganox 1425; Irganox 3114;Irganox 1330; Irganox 1010; Irganox 1076 respectively;2,6-dimethyl-3-hydroxy4-tert-butyl benzyl abietate.

Illustrative examples of skin-core stabilizations are given in EP-A-391438.

Preferred examples of highly stabilizing formulations are thosecontaining more than 0.02%, in particular from 0.04 to 0.2% by weight(with respect to the total weight of polymer and stabilizers) of one ormore antioxidants, like, for example, phenolic antioxidants.

The above stabilizers can be added to the polymer by means ofpelletization or surface coating, or they can be mechanically mixed withthe polymer.

Moreover, the fibers of the present invention can contain otheradditives commonly employed in the art, like anti-slip agents,antistatic agents, flame retardants, fillers, nucleating agents,pigments, anti-soiling agents, photosensitizers.

The fibers of the present invention can be prepared by way of any knownprocess.

In particular, they can be prepared in form of staple fibers, by usingboth long-spinning and short-spinning apparatuses, or by a spun bondprocess, with which the fibers are spread to form directly a fiber weband calendered to obtain a nonwoven article, or by a melt blown process.

The long-spinning apparatuses normally comprise a first spinning sectionwhere the fibers are extruded and air-cooled in a quenching column at arelatively high spinning speed. Subsequently, these fibers go to thefinishing step, during which they are drawn, crimped-bulked and cut.Generally, the above mentioned finishing step is carried out separatelywith respect to the spinning, in a specific section where the fiberrovings are gathered into one single big roving. Said big roving is thensent to drawing, crimping-bulking and cutting apparatuses which operateat a speed ranging from 100 to 200 m/min.

In other types of long-spinning apparatuses the above mentionedfinishing steps are carried out in sequence with the spinning step. Inthis case the fibers go directly from the gathering to the drawingrollers, where they are drawn at a somewhat contained ratio (not greaterthan 1.5) at a speed comparable with that of the spinning step.

The process conditions generally adopted when using the long-spinningapparatuses are the following:

output per hole: greater than 0.2 g/min., preferably from 0.15 to 1g/min., more preferably from 0.2 to 0.5 g/min.;

take up speed: equal to or higher than 500 m/min., preferably from 500to 3500 m/min., more preferably from 600 to 2000 m/min.;

space where the fibers cool off and solidify after exiting the die:greater than 0.50 m. Moreover, it is preferable that the draw ratio befrom 1.1 to 4.

For further details on the long-spinning apparatuses reference is madeto Friedelm Hauser “Plastics Extrusion Technology”, Hauser Publishers,1988, chapter 17.

The short-spinning apparatuses allow for a continuous operation, sincethe spinning speed is compatible with the drawing, crimping and cuttingspeeds.

The process conditions which are best suited to be used according to thepresent invention using short-spinning apparatuses are the following.

The output per hole ranges from 0.005 to 0.18 g/min., preferably from0.008 to 0.07 g/min., more preferably from 0.01 to 0.03 g/min. The takeup speed ranges from 30 to 500 m/min., preferably from 40 to 250 m/min.,more preferably from 50 to 100 m/min. The draw ratios range from 1.1 to3.5, preferably from 1.2 to 2.5. Moreover, the fiber cooling andsolidification space at the output of the die (cooling space) ispreferably greater than 2 mm, more preferably greater than 10 mm, inparticular from 10 to 350 mm. Said cooling is generally induced by anair jet or flow.

For further details on the short-spinning apparatuses reference is madeto M. Ahmed, “Polypropylene fibers science and technology”, ElsevierScientific Publishing Company (1982) pages 344-346.

The spinning temperature for the above long-spinning and short-spinningapparatuses generally ranges from 220° C. to 310° C., preferably from250° C. to 300° C.

The equipment used in the process of spunbonding normally includes anextruder with a die on its spinning head, a cooling tower an air suctiongathering device that uses Venturi tubes. Underneath this device, thatuses air speed to control the take up speed, the filaments are usuallygathered over a conveyor belt, where they are distributed forming a webfor thermal bonding in a calender.

According to the present invention, when using typical spunbondingmachinery, it is convenient to apply the following process conditions.

The output per hole ranges from 0.1 to 2 g/min., preferably from 0.2 to1 g/min.

The fibers are generally cooled by means of an air flow.

The spinning temperature is generally between 210° C. and 300° C.,preferably between 220° C. and 280° C.

The melt blown process uses high velocity hot air to produce fibers ofup to 10 microns in diameter and several centimeters long. Under veryhigh air pressure it is possible to produce fibers as fine as 0.3micron.

Essentially, a polymeric material is passed through an extruder whereheat and pressure cause the polymer to melt. The molten polymer thenenters the melt blowing die and the die-tip orifices which are about 400microns in diameter. The polymer emerging from the orifice is attenuatedby a jet of high velocity hot air. This allows the polymer to maintainits molten state and attenuate until breaking. As the fiber breaks fromthe molten stream, the attenuation air forces it into a stream ofcooling air where the fiber returns from the molten to the solid state.The fiber ultimately lands on the collector wire with the other fibersand forms a homogeneous matt.

Melt blowing can be carried out vertically downwards or horizontallyagainst a rotating surface, to produce basis weights ranging between 5and 1000 g/m².

The spinning temperature used in the melt blowing process is typicallyfrom 260° C. to 350° C.

As previously said, nonwoven articles are obtained directly from thespun bond process. Another known method for producing thermally bondedarticles comprises the production of the staple in a first step,followed by formation of a fiber web by passing the staple fibersthrough a carding machine, and by thermal bonding by calendering(calender rolls are employed).

It has been surprisingly found that the staple fibers of the presentinvention display an unusually high cohesion during the carding step andthe transportation of the obtained web to the calender rolls, so thathigh transportation speeds can be adopted without problems.

The staple fibers can also be thermally bonded by the through airbonding process, where a hot air flow is used to achieve the thermalbonding.

Independently from the specific thermal bonding method employed, thebonding temperatures are preferably within the range from 120° C. to160° C., more preferably from 130° C. to 145° C.

The fibers of the present invention are particularly suited forpreparing thermally bonded articles, in particular nonwoven articles,having optimal mechanical properties and high softness.

The said thermally bonded articles can also be obtained from blends ofthe fibers of the present invention with conventional polyolefin fibers,in particular made of propylene homopolymers.

Moreover, the thermally bonded articles (nonwoven articles) may comprisetwo or more nonwoven layers. Thanks to the use of the fibers of thepresent invention, an improved adhesion among the layers is obtained.

Other thermally bonded articles falling in the definition of the presentinvention are those comprising a nonwoven fabric coupled with apolyolefin film, wherein the nonwoven fabric is made of or comprises thefibers of the present invention, while the polyolefin film may be madeof or comprise the polyolefins described before (for instance the randomcopolymer A) and/or the polyolefin B)).

The coupling between the film and the nonwoven fabric can be obtainedfor instance by heat treatment in a calender or by using adhesives, likehot melts.

The following examples are given to illustrate and not to limit thepresent invention.

The data relating to the polymeric materials and the fibers of theexamples are determined by way of the methods reported below.

MFR: ISO 1133, 230° C., 2.16 Kg;

Melting and crystallization temperature: by DSC with a temperaturevariation of 20° C. per minute;

1-butene content: by IR spectroscopy;

Flexural Modulus: ISO 178;

Tensile Strength at yield: ISO R 527;

Elongation at yield: ISO R 527;

Izod Impact Strength (notched) at 23° C.: ISO 180/1;

Polydispersity Index (PI): measurement of molecular weight distributionof the polymer. To determine the PI value, the modulus separation at lowmodulus value, e.g. 500 Pa, is determined at a temperature of 200° C. byusing a RMS-800 parallel plates rheometer model marketed by Rheometrics(USA), operating at an oscillation frequency which increases from 0.01rad/second to 100 rad/second. From the modulus separation value, the PIcan be derived using the following equation:

PI=54.6×(modulus separation)^(−1.76)

wherein the modulus separation (MS) is defined as:

MS=(frequency at G′=500 Pa)/(frequency at G″=500 Pa)

wherein G′ is the storage modulus and G″ is the low modulus.

Fractions soluble and insoluble in xylene at 25° C.: 2.5 g of polymerare dissolved in 250 ml of xylene at 135° C. under agitation. After 20minutes the solution is allowed to cool to 25 ° C., still underagitation, and then allowed to settle for 30 minutes. The precipitate isfiltered with filter paper, the solution evaporated in nitrogen flow,and the residue dried under vacuum a 80° C. until constant weight isreached. Thus one calculates the percent by weight of polymer solubleand insoluble at room temperature (25° C.).

Titre of fibers: from a 10 cm long roving, 50 fibers are randomly chosenand weighed. The total weight of the said 50 fibers, expressed in mg, ismultiplied by 2, thereby obtaining the titre in dTex.

Tenacity and Elongation (at break) of fibers: from a 500 m roving a 100mm long segment is cut. From this segment the single fibers to be testedare randomly chosen. Each single fiber to be tested is fixed to theclamps of an Instron dinamometer (model 1122) and tensioned to breakwith a traction speed of 20 mm/min. for elongations lower than 100% and50 mm/min. for elongations greater than 100%, the initial distancebetween the clamps being of 20 mm. The Ultimate strength (load at break)and the Elongation at break are determined.

The Tenacity is derived using the following equation:

Tenacity=Ultimate strength (cN)×10/Titre (dTex)

Bond strength of fibers: specimens are prepared from a 400 Tex roving(method ASTM D 1577-7) 0.4 meter long, made up of continuous fibers.After the roving has been twisted eighty times, the two extremities areunited, thus obtaining a product where the two halves of the roving areentwined as in a rope. The thermal bonding is carried out on saidspecimen using a Bruggel HSC-ETK thermal bonding machine, operating atvarious plate temperatures (see in the tables) using a clamping pressureof 0.28 MPa and 1 second bonding time. The previously said dynamometer,operated at a traction speed of 2 cm/min., is used to measure theaverage force required to separate the two halves of the roving whichconstitute each specimen at the thermal bonding point. The obtainedgraph shows the force varying from minimum to maximum values (peaks areobtained). The value resulting from averaging out all the minimum andmaximum values shown in the graph represents the said average force. Theresult, expressed in cN, is obtained by averaging out at least eightmeasurements, and represents the bond strength of the fibers.

In alternative, when nonwoven samples are prepared, the bond strength isdetermined on specimens 20 cm long and 5 cm wide. The 5 cm wideextremities are fixed to the clamps of the dynamometer and tensioned ata clamp speed of 100 mm/min. (the initial distance between the clampsbeing of 10 cm). The maximum force measured in the Machine Direction(MD) and in the Cross Direction (CD), with respect to the calenderingstep, represents the strength of the fibers.

Softness of fibers: specimens are prepared from a 400 Tex roving 0.6 mlong, made up of continuous fibers. The extremities of the roving arefixed to the clamps of a twist measuring device (Torcimetro Negri eBossi S.p.A., Milano) and subjected to 120 runs twist. The twistedroving is taken off and the two extremities are united, thus obtaining aproduct where the two halves of the roving are entwined as in a rope.The so obtained specimens are bent double and the extremities are fixedbetween the two parallel rolls of a Clark softness tester, leaving adistance of 1 cm between the two halves of the specimen.

Then the two rolls of the tester are jointly rotated rightward andleftward until the specimen reverses its bending direction each time dueto the rotation of the plane on which the two rolls lie. The height ofthe specimen above the two rolls is adjusted so to have the sum of thetwo angles of plane rotation equal to 90°. The specimen is taken out,cut to the said height and weighed.

The softness value is derived from the following equation:

Softness=(1/W)×100

where W is the weight, in grams, of the specimen cut to the said height.

POLYMERS SUBJECTED TO SPINNING Polymers I and Ib

Propylene/1-butene crystalline random copolymers obtained bycopolymerizing the monomers in the presence of a high yield, highlystereospecifc Z-N catalyst, and having the following properties:

Polymer I Polymer Ib MFR (dg/min.):  10.6   1.8 Xylene insoluble at 25°C. (% by weight):  97.6  98.1 Melting temperature (° C.): 141  146Crystallization temperature (° C.):  91  93 1-butene content (% byweight):  8.3   6.1 PI:  4   3.87 Flexural Modulus (MPa): 950 1250Tensile Strength at yield (MPa):  27  28 Elongation at yield (%):  12 10 Izod Impact Strength (notched) at 23° C.  4   8.1. (KJ/m²)

To the said Polymers I and Ib 0.04% by weight of sodium stearate and0.15% by weight of Irganox B 215 are added by means of pelletization. Aparaffinic oil (0.05% by weight with respect to the total weight ofpolymer and additives) is also added as a dispersing agent for the saidadditives.

Irganox B 215 is a blend of ⅓ by weight of Irganox 1010 and ⅔ by weightof Irgafos 168.

Polymer Ib is not used as such for spinning.

Polymer II

Obtained by chemical degradation of Polymer I with 0.021% by weight ofLuperox 101.

The resulting MFR and PI values are 25.8 dg/min. and 3 respectively.

Polymers III and IV

Obtained by chemical degradation of Polymer Ib with 0.073% by weight(Polymer III) and 0.038% by weight (Polymer IV) of Luperox 101.

The resulting MFR and PI values are respectively 26.8 dg/min. and 2.36for Polymer III, and 12.5 dg/min. and 2.79 for Polymer IV.

Propylene homopolymers

All the comparative examples are carried out by spinning propylenehomopolymers having the MFR and PI values reported in the tables. Allthe homopolymers contain about 96% by weight of a fraction insoluble inxylene at 25° C.

SPINNING AND CALENDERING APPARATUSES

In all the examples, except for Examples 5,5c, 6 and 6c, a Leonard 25spinning pilot line with length/diameter ratio of the screw of 5 (builtand marketed by Costruzioni Meccaniche Leonard-Sumirago (VA)) is used.

In Examples 5 and 5c a semi industrial short-spinning line is used,witha spinneret having 65000 holes and a central quenching air device(quenching temperature: about 19° C.).

In Examples 6 and 6c a high speed carding/calendering plant is used.

The maximum speed values reported in the following tables are thehighest take up speeds at which a reduced number of fibers is brokenafter 30 minutes (this number is given in the tables as “No of breaks atmax. speed/30′”).

EXAMPLES 1 AND 2 AND COMPARISON 1C AND 2C

It is operated under the long-spinning conditions reported in Table 1.

The space between the exit of the die and the point at which thefilaments come into contact with the quenching air is of 10 cm.

The fibers of Examples 1 and 2 are obtained by spinning the above saidPolymer 1, while those of Comparison Examples 1c and 2c are obtained byspinning homopolymers having a skin-core stabilization, as demonstratedby the sensibly increased MFR values in the spun fibers (fiber MFR).

The characterization of the fibers so obtained is reported in Table 1 aswell.

TABLE 1 Example No. 1 2 1c 2c Polymer dg/min 10.6 10.6 18.8 12.0 MFR PI4.0 4.0 3.95 3.94 Head T ° C. 260 265 270 280 T melt ° C. 267 273 278293 Head Bar 36 35 25 38 pressure Hole mm 0.4 0.4 0.4 0.4 diameterOutput per g/min 0.4 0.4 0.4 0.4 hole n. holes in u 61 61 61 61 the dieQuenching T ° C. 24.6 23.4 21.6 20.0 Take up m/min 1500 1500 1500 1500speed Fiber MFR dg/min 17.8 18.8 87 94 Maximum m/min 3900 3900 3900 3900speed No. of u 0 1 5 1 breaks at max. speed/ 30′ ONLINE ORIENTATION Iroll speed m/min 1500 1500 1500 1500 I roll temp- ° C. 50 50 50 50erature II roll speed m/min 2250 2250 2250 2250 II roll temp- ° C. 110110 110 110 erature III roll speed m/min 2250 2250 2250 2250 III rolltemp- ° C. 90 90 90 90 erature Draw ratio 1:1.5 1:1.5 1:1.5 1:1.5ORIENTED FIBER CHARACTERIZATION Titre DTex 2.35 2.10 1.95 2.00 TenacitycN/Tex 26 27.9 18.2 19.2 Elongation % 235 230 350 395 Softness 1/g 850750 Bond CN 895 ± 110 995 ± 135  540 ± 150 380 ± 69 strength (150° C.)Bond CN 630 ± 110 540 ± 115 295 ± 51 — strength (145° C.) Bond CN 315 ±40  315 ± 35  20 ± 17 — strength (140° C.)

Notes

head T and head pressure are the temperature and pressure measured onthe spinning head;

for the bond and strength measurements, the temperature at which thermalbonding occurred is given between brackets.

EXAMPLES 3 AND 4 AND COMPARISON 3c AND 4c

It is operated under the long-spinning conditions reported in Table 2.

The space between the exit of the die and the point at which thefilaments enter into contact with the quenching air is of 10 cm.

The fibers of Examples 3 and 4 are obtained by spinning the above saidPolymer IV, those of Comparison Examples 3c and 4c by spinning propylenehomopolymers having a skin-core stabilization and a strongerstabilization (for spun bonding) respectively.

The characterization of the fibers so obtained is reported in Table 2 aswell.

TABLE 2 Example No. 3 4 3c 4c polymer MFR dg/min 12.5 12.5 12.0  12.3 PI2.79 2.79 3.92   2.65 head T ° C. 270 280 280  285 T melt ° C. 277 287290  292 head pressure Bar 28 24 29  26 hole diameter mm 0.4 0.4 0.4  0.4 output per hole g/min 0.4 0.4 0.4   0.4 n. holes in the die u 6161 61  61 quenching T ° C. 23.6 24.5 17.0 take up speed m/min 1500 15001500 1500 fiber MFR dg/min 18 20.5 75  19.4 maximum speed m/min 42004500 4200 2700* No. of breaks at max. u 1 0 0   0 speed/30′ ONLINEORIENTATION I roll speed m/min 1500 1500 1500 1500 I roll temperature °C. 50 50 50  50 II roll speed m/min 2250 2250 2250 2250 II rolltemperature ° C. 110 110 110  110 III roll speed m/min 2250 2250 22502250 III roll temperature ° C. 90 90 90  90 Draw ratio 1:1.5 1:1.5 1:1.51:1.5 ORIENTED FIBER CHARACTERIZATION Titre dTex 1.95 1.8 2.20   1.85Tenacity cN/Tex 48.6 55.3 20.7  36.1 Elongation % 110 105 350  150Softness 1/g 1030 1055 795 Bond strength (150° C.) cN 315 310 350  170*7 breaks at 3000 m/min in 10 minutes

EXAMPLES 5 AND COMPARISON 5c

In Example 5 the above said Polymer I is spun into fibers by operatingwith a first Godet speed of 108 m/min., a second Godet speed of 134m/min., an output of 90 Kg/h, and a head temperature of 310° C. Nospinneret fouling occurred, and no output limitation was evidenced.

In Comparison Example 5c the same propylene homopolymer as in ComparisonExample 2c is spun into fibers by operating with a first Godet speed of103 m/min., a second Godet speed of 134 m/min., an output of 90 Kg/h anda head temperature of 320° C.

The draw ratios and the characterization of the fibers so obtained arereported in Table 3.

TABLE 3 Example No. 5 5c Draw ratio 1.24 1.3 TITRE dTex 2.35 2.42Tenacity cN/tex 22 20 Elongation % 240 300 Bond strength at 150° C. Cn —250 Bond strength at 140° C. cN 1135 150 Bond strength at 135° C. cN 765— Bond strength at 130° C. cN 410 —

EXAMPLES 6 AND COMPARISON 6c

The fibers of Example 5 and Comparison Example 5c are thermally bondedin Example 6 and Comparison Example 6c respectively, bycarding/calendering under the conditions reported in Table 4, therebyobtaining 20 g/m² nonwovens.

The Tenacity values of the so obtained nonwovens are reported in Table 4as well.

TABLE 4 Calendering Conveyor Example temperature belt speed MD TenacityCD Tenacity No. ° C. m/min N/5 cm N/5 cm 6 137 140 55 5.4 6c 155 140 404.0

EXAMPLES 7-14 AND COMPARISON 7-10c

It is operated under the spun bonding conditions reported in Tables 5 to7.

The fibers of Examples 7-14 are obtained by spinning the followingpolymers:

Example Polymer  7 I  8 III  9 III 10 III 11 II 12 IV 13 IV 14 IV

In Comparison Examples 7c-10c propylene homopolymers with a spun bondingstabilization are used.

TABLE 5 Example No 7 7c 8c Polymer MFR Dg/min 10.6 12.0 23.9 PI 4.0 2.742.58 Head T ° C. 270 285 250 T melt ° C. 277 292 258 Head pressure Bar27 24 21 Hole diameter Mm 0.6 0.6 0.6 Output per hole g/min 0.6 0.6 0.6n. holes in the die u 37 37 37 Quenching T ° C. 23.1 22.4 20.6 Take upspeed m/min 1500 1500 1500 Fiber MFR Dg/min 20.7 17.8 33.5 Maximum speedm/min 4200 4200 4500 No. of breaks at max. U 2 3 2 speed/30′ Take upspeed m/min 3600 3600 3600 Titre single fiber Dtex 1.8 1.75 1.75Elongation % 315 325 280 Tenacity CN/Tex 21.9 23.6 21.2 Softness 1/g1150 900 925 Bond strength (150° C.) CN — 150 ± 25 180 ± 20 Bondstrength (140° C.) CN 920 ± 70 — —

TABLE 6 Example No 8 9 10 11 9c Polymer MFR dg/min 26.8 26.8 28.8 25.823.9 PI 2.36 2.36 2.36 3.0 2.58 Head T ° C. 240 250 230 250 250 T melt °C. 249 258 237 258 258 Head pressure Bar 22 20 25 21 21 Hole diameter mm0.6 0.6 0.6 0.6 0.6 Output per hole g/min 0.6 0.6 0.6 0.6 0.6 n. holesin the u 37 37 37 37 37 die Quenching T ° C. 24.1 24.3 24.1 20.6 20.6Take up speed m/min 1500 1500 1500 1500 1500 Fiber MFR dg/min 32.2 33.632.5 32.9 33.5 Maximum m/min 4200 4200 4200 4200 4500 speed No. ofbreaks u 1 3 1 3 2 at max. speed/ 30′ Take up speed m/min 3600 3600 36003600 3800 Titre single dTex 1.75 1.80 1.95 1.90 1.75 fiber Elongation %230 140 235 275 280 Tenacity cN/Tex 23.4 20.1 20.6 20.3 21.2 Softness1/g 1085 — 1010 925 Bond strength cN Mold — — — 180 (150° C.) Bondstrength cN 675 — — — — (145° C.) Bond strength cN 290 — — 850 — (140°C.)

TABLE 7 Example No 12 13 14 10c Polymer MFR dg/min 12.5 12.5 12.5 12.0PI 2.79 2.79 2.79 2.74 Head T ° C. 285 270 280 285 T melt ° C. 291 277287 292 Head pressure Bar 18 28 24 24 Hole diameter Mm 0.6 0.4 0.4 0.6Output per hole g/min 0.6 0.4 0.4 0.6 n. holes in the die U 37 61 61 37Quenching T ° C. 24.8 23.6 24.1 22.4 Take up speed m/min 1500 1500 15001500 Fiber MFR dg/min 27.9 17.1 20.9 17.8 Maximum speed m/min 4200(4500) 4200 4500 4200 No. of breaks at max U 0(5) 1 0 3 speed/30′ Takeup speed m/min 3600 3600 3600 3600 Titre single fiber DTex 1.75 1.151.15 1.75 Elongation % 210 220 200 325 Tenacity cN/Tex 25.2 30.8 31.823.6 Softness 1/g 1085 1045 1115 900 Bond strength (150° C.) CN 930 — —150 ± 25 Bond strength (145° C.) CN 605 — — — Bond strength (140° C.) CN255 — — —

EXAMPLES 15-22 AND COMPARISON 11c

Further spinning tests were performed in the Leonard 25 spinning pilotline with length/diameter ratio of the screw of 5 (built and marketed byCostruzioni Meccaniche Leonard-Sumirago (VA)) in the typical conditionsfor thermal Bonding staple. Online orientation adopted is the typicalstretch ratio for Hygiene applications.

A homopolymer for thermal bonding staple, having PI=3.91, MFR=11.6 andXylene soluble 4.1% wt, and a typical additive package to induceskin/core structure in the filament, was spun in pure as reference. Themain conditions are reported in Table 8.

The random copolymer is the Polymer I previously described and has atypical additive package for thermal bonding staple (to induce skin/corestructure in the filament).

It was tested in dry blend with the said homopolymer in differentpercentage (spinning Examples N. 15-22) and in pure (Ex. N. 11c). Intable 8 are reported all the results. The blends were spun at lowertemperature (270° C. vs. 280° C.) due to the lower melting temperatureof the random copolymer.

In particular, Softness, Bonding strength, Fibre Tenacity increase withthe amount of random copolymer. Surprisingly, even at 2% wt. of randomcopolymer the blend exhibits a sudden rise of the properties.

Elongation is lower the higher the Tenacity due to the higher filamentorientation induced by the random copolymer.

Spinnability is fully suitable for the application in all the cases.

A Thermofil internal test apparatus is used to measure the filamentretraction at a selected temperature (generally 130° C.).

The filament is clamped without any pretension imposed and placed at130° C. for 600 seconds.

The variation of the length (usually contraction) in percentage withrespect to the initial length amounts to the retraction.

TABLE 8 Example N. 11c 15 16 17 18 19 20 21 22 Polymer I amount % wt 0 25 10 15 20 30 50 100 polymer MFR dg/min 11.6 11.6 11.6 11.6 11.6 11.311.0 11.0 10.7 PI 3.91 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 head T ° C. 280270 270 270 270 270 270 270 270 T melt ° C. 290 281 280 280 280 280 280279 280 head pressure Bar 24 29 29 30 31 29 30 32 32 hole diameter mm0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 output per hole g/min 0.4 0.4 0.40.4 0.4 0.4 0.4 0.4 0.4 n. holes in the die u 61 61 61 61 61 61 61 61 61quenching T ° C. 17.7 19.3 19.5 19.9 18.5 18.2 18.3 19.8 21.6 take upspeed m/min 1500 1500 1500 1500 1500 1500 1500 1500 1500 fiber MFRdg/min 109 60.4 56.9 57.2 58.2 — — 62 72 maximum speed m/min 3900 36003900 3600 3600 3900 3600 3900 4200 No. of breaks at max. u 3 2 3 2 1 0 01 0 speed/30′ ONLINE ORIENTATION I roll speed m/min 1500 1500 1500 15001500 1500 1500 1500 1500 I roll temperature ° C. 50 50 50 50 50 50 50 5050 II roll speed m/min 2250 2250 2250 2250 2250 2250 2250 2250 2250 IIroll temperature ° C. 110 110 110 110 110 110 110 110 110 III roll speedm/min 2250 2250 2250 2250 2250 2250 2250 2250 2250 III roll temperature° C. 90 90 90 90 90 90 90 90 90 Draw ratio 1:1.5 1:1.5 1:1.5 1:1.5 1:1.51:1.5 1:1.5 1:1.5 1:1.5 ORIENTED FIBER CHARACTERIZATION Titre dTex 1.952.0 2.0 1.90 1.85 2.00 2.00 1.90 1.9 Tenacity cN/Tex 20 20.1 23.5 25.525.1 26.1 28.1 31.0 34.0 Elongation % 225 300 310 270 270 250 200 245145 Softness 1/g 750 905 1010 975 975 1000 920 960 1030 Bond strength(150° C.) cN 620 ± 100 750 ± 110 730 ± 135 780 ± 193 820 ± 150 850 ± 227920 ± 227 930 ± 320 1010 ± 227 Retraction at 130° C. % 6 6 6 6 6.5 7.07.5 8.0 10

Polymer I was used pure and in blend in Short spinning process toproduce staple for Hygiene.

Staple was thermobonded at different calendering temperatures(Temp.(C)-1=Flat roller temp.), Temp.(C)-2=Embossing roller temp.) incomparison with homopolymers staple produced by Long spinning process(much more effective to produce Skin/Core filament structure/enhancethermobondability than the Short spinning).

Line speed 80m/min (speed of Machine direction web production)

A. 100% Polymer I by Short Spinning Staple

Temp. (C)-1 Temp. (C)-2 Web Wt. (g/m²) MD (Kg) MD Elong. (%) CD (Kg) CDElong. (%) 155 154 21.8 3.12 34.5 0.92 67.5 155 149 21 3.14 49.3 0.9494.6 152 146 22 3.81 49.1 0.87 96.1 149 143 22.4 4.45 67.6 0.86 100.2146 140 22.8 4.49 74.6 0.66 84.6

B. 70% Polymer I+30% homopolymer by Short Spinning Staple

Temp. (C)-1 Temp. (C)-2 Web Wt. (g/m²) MD (Kg) MD Elong. (%) CD (Kg) CDElong. (%) 164 158 21.9 3.59 49.7 0.87 92.8 161 155 21.7 4.01 58.2 0.94113.1 158 152 21.2 4.06 66.3 0.79 103.4 155 150 22 3.79 65.7 0.84 123.3152 146 21.2 3.81 71.5 0.53 83.1

C. 50% Polymer I+50% homopolymer by Short Spinning Staple

Temp. (C)-1 Temp. (C)-2 Web Wt. (g/m²) MD (Kg) MD Elong. (%) CD (Kg) CDElong. (%) 152 146 21 3.23 65.5 0.36 65.5 155 150 21.1 3.71 69.8 0.6289.2 158 152 22 3.69 58.4 0.84 108.9 161 155 21.5 3.69 55.3 0.81 109.6164 158 21.7 3.69 46.9 0.76 87.9

D. 100% homopolymer by Short Spinning Staple ref N.1

Typical values Web Wt. (g/m²) MD (Kg) MD Elong. (%) CD (Kg) CD Elong.(%) 21 3.3 90.0 0.7 80

E. 100% homopolymer by Long Spinning Staple ref. N.2

Typical values Web Wt. (g/m²) MD (Kg) MD Elong. (%) CD (Kg) CD Elong.(%) 21 3.6 90.0 1.0 85

Staple fibres produced by Short Spinning process using Polymer I pure orin blend with homopolymer can compete with Long Spinning Staple fibres(more expensive and delicate process) during the web preparation bycarding thermobonding.

What is claimed is:
 1. Thermal bondable polyolefin fibers comprising 1%by weight or more of a random copolymer A) of propylene with one or morecomonomers selected from α-olefins of formula CH₂═CHR, wherein R is aC₂-C₈ alkyl radical, the amount of said comonomer or comonomers beingfrom 3% to 20% by weight with respect to the total weight of the randomcopolymer A).
 2. The fibers of claim 1, having Tenacity values equal toor higher than 10 cN/Tex.
 3. The fibers of claims 1 or 2, comprising apolyolefin composition C) containing from 20% to 100% by weight of therandom copolymer A) and from 0% to 80% by weight of a polyolefin B)selected from polymers or copolymers, and their mixtures, of CH₂═CHRolefins where R is hydrogen or a C₁-C₈ alkyl radical.
 4. The fibers ofclaim 1, obtained from a random copolymer A) having a value of TensileStrength at yield equal to or higher than 24 Mpa, or from a polyolefincomposition comprising such copolymer A).
 5. The fibers of claim 1,obtained from a polymeric material resulting from the chemicaldegradation of a random copolymer A) having a value of Tensile Strengthat yield equal to or higher than 24 Mpa, or from the chemicaldegradation of a polyolefin composition comprising such copolymer A). 6.The fibers of claim 1, in form of single or composite fibers.
 7. Aprocess for preparing the fibers of claim 1, by spinning the randomcopolymer A), or by spinning a polyolefin composition comprising 1% byweight or more of such copolymer A).
 8. Thermally bonded articlescomprising the fibers of claim
 1. 9. The thermally bonded articles ofclaim 8, in form of nonwoven articles.
 10. The nonwoven articles ofclaim 9, comprising two or more nonwoven layers.
 11. The thermallybonded articles of claim 8, comprising a nonwoven fabric coupled with apolyolefin film.
 12. The fibers of claim 2 further comprising apolyolefin composition C) containing from 20% to 100% by weight of therandom copolymer A) and from 0% to 80% by weight of a polyolefin B)selected from polymers or copolymers, and their mixtures, of CH₂═CHRolefins where R is hydrogen or a C₁-C₈ alkyl radical.